Many of the high powered scopes come with AO. I have an AO on my Burris 6-18x, and I set it whatever distance I am shooting, when I am at the range. When I am hunting, I set it to 150 yards and leave it alone.

An AO comes in REALLY handy on a rimfire. Some models you can set for distances as close as 10 yards. A local indoor range only goes as far as 25 yards, so being able to focus the scope at such a clost range is a very nice feature. There are a few folks that shoot their centerfire rifles, with cast bullets at reduces velocities, in the indoor range. Many of them use a scope with an AO.